Method for manufacturing cold rolled steel excellent in press-formability

ABSTRACT

This invention relates to a method for producing cold rolled low carbon rimmed or capped steel having excellent press-formability, e.g., deep drawability when cold rolled.

United States Patent 1191 Tenmyo e11 a1.

[ 1 Dec. 10, 1974 1 1 METHOD FOR MANUFACTURING COLD 2.878.151 3/1959 1361111 et111. 1411/12 ROLLED STEEL EXCELLENT [N 3,178,318 4/1965 Shimizu et :11. 1 1 1 148/2 3,244,565 4/1966 Mayer et a1 1 1 148/12 PRESS-FORMABILITY 3,248,270 4/1966 Laidman eta1.. 148/12 [75] v Inventors; Ge'nnosuke Tenmyo, Nippon; K 0 3,323,953 6/1967 Lesney 1 148/39 Yamada, Kawasaki Shi, both f 3,335,036 8/1967 Yoshida et a1 148/12.l

Ja an FOREIGN PATENTS OR APPLICATIONS 1731 Asslgnee: N'ppon Kkan Kabush'k' 1,027,158 4/1966 Great Britain l48/12.1 Tokyo Japan 1,117,322 6/1968 Great Britain 148/12.1 [22] Filed: May 30, 1972 1 [211 Appl No; 257,982 Primary Examiner-W. W. Stallard Attorney, Agent, or Firm-F1ynn & Frishauf Related U.S. Application Data [63] Continuation of Ser. No. 876,347, Nov. 13, I969,

abandond- [57] ABSTRACT 30 Forei A lication Priorit Data 1 J gn pp y 43 82864 This invention relates to a method for producing cold Nov. 14, 1968 Japan rolled low carbon rimmed or pp Steel having ex 52 U.S. c1. 148/121 zgig g f g deep drawabmy when [51] Int. Cl C2111 9/48 of Search 1 5 C i s 1 i g gu e [56] References Cited UNITED STATES PATENTS 2,736,648 2/1956 Eckel 75/123 F Ar FArIFArFAr'r/ir A 1.820.551.92 .95 0 '98 0.1 .79 0.12 B 1.77 0.70 1.76 006 .78 0,601.76 0.40159 0.25

jg 2.4 1 f 2.3 22 [1 I1 I, 5 1 1 1 2.0 1 1 1 1 1 1 1 g 1.9 I 1 '1 1.8 1 1 1 1 g 1 7 1 1 1 1 1' 1 1 1 1 1 1 1 1. 6 1 H l a! 1.5 1" 11 Q4590 04590 04590 04590 04590 D1RECT1ON 8O 84 "/6 REDUCTION PAIENTELUEEIOIQM 3.853.636

FIGI

T A L82 055 L92 0441.95 0.26.98 010?.79 012 B I.77|0.70 L76 0.06 L78 0.66 L76 0.40 I69 0.25

I I I I I I I I I I I I I I I I I (545 90 04590 O 4590 04590 04590 D|RECTION 65.3 7L5 75 8O 84 VOREDUCTION METHOD FOR MANUFACTURING COLD ROLLED STEEL EXCELLENT IN PRESS-EOIRMABILITY This is a continuation of application Ser. No. 876,347, filed Nov. l3, 1969, now abandoned.

Formability and shapability are usually required in cold rolled steel for subsequent press-working. Usually, formability and shapability are determined by the resulting value of mechanical properties such as yield point, eleongation, Erichsen value, conical cup value and Lankford value, e.g., normal anisotropy and the like. The values affect significantly the qualities of the steel for press'working. Various methods have been proposed to improve the value of these properties. However, according to the known methods, the improvement of these properties is limited considerably. For instance, orange peel which is well known is caused by enlarging the grain size. Generally, the Lankford value of cold rolled steel sheet acting for deep-drawability shows heavy orientation, and such deep-drawability is determined by 7, that is, the average value of r", r, and r which are the Lankford values at 45 and 90 to-the rolling direction. 7is obtained from the following formula:

However, in the actual press-working, said deep-drawability of steel sheets cannot be determined only by the value of r, which quality is assumed to result from Ar, i.e., planar anisotropy, in many cases. Planar plastic anisotropy Ar is obtained by the following formula:

In an ordinary cold rolled steel, r is the highest, and the lowest. It happens frequently, as known from experience, that while r is not high, I and r become higher so that the average value 7 is higher. In such cases, any press-forming subject to r does not assist in an improvement in said deep-drawability.

When the Lankford value is raised in the prior art, Ar also tends to be high, that is to say, the actual deep-drawability is not improved in proportion to the average Lankford value. a

Two methods for the improvement of press-formability using rimmed steel (or capped steel) are known in the prior art. These methods are decarburizing annealing and two stage cold reducing containing intermediate and final annealing.

However, there exists a limit to the mechanical properties of cold rolled steel obtained by these methods. For instance, the average Lankford value Tis L8 in case of decarburizing annealing process, and it is 2.0 in

case of two-stage cold reducing process.

In one method, the cold reducing rate is made exceptionally high or exceptionally low in order to lessen said planar anisotropy. This method results in neglecting deep-drawability of the steel. A method using an adequate combination of cold reducing rates by means of a two-stage cold reducing process leads to products with good deep-drawability and small Ar, but it is expensive on a production basis.

Cold rolled steels having excellent press-formability are more and more needed in the fields of automobile manufacture and other industries. However, cold rolled steels produced by theprior methods cannot meet these needs.

This invention provides a novel method of reducing or eliminating the above defects of the prior art methods and of meeting the very strict requirements in these fields.

With a knowledge of the limitations of cold rolled steel obtained by known methods, we have studied the effect of the components of cold reducing rimmed steel (or capped steel). We have established that manganese exerts a remarkable effect upon the properties of cold rolled steel, particularly the Lankford value, and we have developed a method for producing cold rolled steel having properties unobtainable by known methods, by means of a combination. of the most suitable content of Mn, which ranges from 0.25 to 0.03 by weight, and the most suitable requirements of cold reducing and annealing for the above steel.

An object of this invention is to provide a method of producing a cold rolled steel with excellent press-formability and yet without orange peel and having a higher Lankford value. Another object of the present invention is to provide a method of producing a cold rolled steel having a smaller planar anisotropy while keeping a higher normal anisotropy.

The method of the invention utilizes a rimmed or capped steel having a manganese content ranging from 0.25 to 0.03.

The upper limit of the manganese content is set at 0.25 percent because a greater manganese 7 content does not improve the properties of the products. The lower limit is set at 0.03 percent because it is impossible to make such steels, containing less than this amount, on an industrial scale. his not necessary to define the content of components other than the manganese, and the content is enough to be within a wellknown range of rimmed steel. It is not necessary to limit the carbon content in steel, if it is to be removed from the steel by decarburizing annealing in the subsequent manufacturing process. We have found that the lower the sulphur content, the better the properties of the cold rolled steel. However since it does not affect the objects of this invention, if the sulphur is present in a small amount as an ordinary impurity, it is not necessary to limit the sulphur content. Oxygen affects the grain growth of the cold rolled steel, but the oxygen amount corresponding to the above carbon and managnese amounts in rimmed or capped steel is acceptable for the purposes of this invention. Therefore it is not necessary to define the amount with any particularity. This is also true in respect of the other impurities.

For a better understanding of the invention reference will now be made to the accompanying drawings, in which:

The single FIGURE is a series of graphs showing the relationship between the cold reducing rate and the Lankford value.

In carrying out the present invention a hot rolled coil is made of the above steel with known hot rolling process. One of the two methods or processes of cold reducing and annealing is selected based on the properties of the desired final product.

The first method is aimed at producing a cold rolled steelhaving an extremely high Lankford value, wherein two stage cold reducing and annealing process are emloyed basically. The hot rolled coil is processed in a first cold rolling, an intermediate annealing, a second cold rolling and a final annealing, successively. After this, temper rollingis, if necessary, carried out. Either the intermediate annealing or the final annealing of a temperature ranging from 630C to 850C is performed in decarburizing or decarburizing-denitrifying atmosphere, and the other in the usual reducing atmosphere.

The Lankford value of the products varies according to the combination of the first and second reduction rates. In a desirable combination, the value of the first cold reducing rate is more than 30 percent, and that of the second cold reducing rate is more than 50 percent.

The second method is used for producing cold rolled steel having a larger normal anisotropy and small planaranisotropy. This cold reducing is performed atonly reference to EXAMPLE 1 Rimmed steel having a chemical composition as shown in Table l is manufactured in a basic oxygen furnace.

one stage.

Either decarburizing or decarburizing-deni'trifying TABLE 1 atmosphere in annealing temperature of 630C to 850C is chosen as occasion calls. Temper rolling is carried out to finish the products, if this is necessary. We] C P S N 0 As the cold reducing rate increases, which is mentioned A 0.1] 012 0016 0010 (10014 0029 hereinafter, the planar anisotropy changes (see the sin- B 009 015 0012 00013 (1035 gle drawing) When the cold reducing rate is set at more than 65 percent, r which is the Lankford value'in the direction of 45 to rolling direction consequently becomes higher and finally becomes higher than r" and The above steel ingot is rolled into slab. The finishing the planar anisotropy Ar becomes very small. This relatemperature for hot rolling of this slab is 850C-870C tion is obtained only with chemical compositions A, esand the coiling temperature is 550C-580C. The pecially Mn content, employed in the present inventhickness of the hot rolled coil is 7.0-3.8 mm. tion. With known steels B processed-by similar steps, A cold rolled steel sheet of 0.8mm final thickness is such as a result can be obtained only when the cold reproduced with the manufacturing process shown in ducing rate is very high, that is more than 85 percent. Table 2. The mechanical properties of the products are It is difficult to get such a high cold reducing rate on an shown in Table 3.

TABLE 2 No. thickness lst temp. of 2nd temp. of thickness steel of hot rollcold reintermedicold refinal of products e ed coil duction ate duction annealing mm rate annealing rate C mm 1 A 7.0 54.3 700C 75.0 *770C 0.8 2 do. 5.0 46.7 700C 71.5 *770C 0.8 3 do. 5.0 68.0 700C 50.0 *770C 0.8 4 do. 3.8 58.0 700C 50.02 770C 0.8 5 B 6.0 61.7 720C 65.3 "800C 0.8 6 do. 6.07 61.7 *750C 65.3 800C 0.8

(' processed by dccarburizing annealing) TABLE 3 No yield tensile elongation Erichsen conical grain point strength value cup size kg/mm kg/mm mm value ASTM 1 14.2 26.6 56.5 12.6 through 6.5

drawing 2 14.0 27.7 56.3 12.0 through 6.8

drawing 3 15.0 26.9 56.3 12.5 35.8 7.8 4 16.5 27.4 55.4 12.2 36.4 8.0 5 12.8 26.6 55.7 13.1 through 6.7

drawing 6 14.0 27.1 55.0 13.1 35.1 7.0

of direction is high when the cold reducing rate is in the range of percent, which is an industrially optimum range for cold rolling.

The relation between normal anisotropy, their average values and the planar anisotropy are shown in Table 4.

TABLE 4 Steel r r r 1" Ar Even tho ug h it wasaneealed at a very high temperature, its grain size is ASTM 6 or 7 and there was no orange peel.

EXAMPLE 2 A slab of rimmed steel being composed of 0.09% C, 0.18% Mn, 0.01% P, 0.008% S and 0.036% 0 was used to make a hot rolled coil 2.3 5.0 mm in thickness. Its finishing temperature was 840 870C and its coiling temperature was 530-580C. This hot rolled coil was cold rolled to 0.8 mm in thickness and then decarburizing annealing was done at 780C as a maximum. There after the necessary temper rolling was carried out.

Thus, a cold rolled steel having excellent mechanical properties and an average Lankford value Twas obtained, in which the Lankford value at to the rolling direction is remarkably high and planar anisotropy Ar is small, at the reducing rate of 65 to 85 percent. In the case of this example, grain size is minute and there was no orange peel.

The invention hereinabove described first leads to an extremely high Lankford value, secondary leads to very small planar anisotropy while keeping high Lankford value. Further, excessive grain-growth or orange peel is rarely seen. These features shown an excellent formability and shapabilityof this invention steel. Thus it has advantages from both the technical and economic viewpoints. The same effect can be expected by using capped steel instead of rimmed steel.

What is claimed is:

l. A method of producing a cold rolled low carbon rimmed or capped steel having a low planar anisotropy (Ar) and a high normal anisotropy and having a manganese content in the range of 0.03 to 0.25 percent, comprising: cold reducing with a reduction rate of from 75 to 84 percent, and then annealing at a temperature within the range of 630C to 850C and in a decarburizing or decarburizing-denitrifying atmosphere.

2. A method as claimedin claim 1, wherein after annealing the steel is subjected to temper rolling.

3. The method of claim 1 wherein said reduction rate is 75 percent.

4. The method of claim 1 wherein said reduction rate is 80 percent.

5. The method of claim 1 wherein said reduction rate is 84 percent.

TABLE 5 (reducing rate 75%) steel yield tensile elongation Erichsen conical grain point strength value cup growth kg/mm mm value ASTM kg/mm mm steel of this 14.9 27.3 52.4 12.3 36.0 7.5

invention steel of the prior 16.2 27.5 52.3 11.5 36.6 6.9 an 

1. A METHOD OF PRODUCING A COLD ROLLED LOW CARBON RIMMED OR CAPPED STEEL HAVING A LOW PLANAR ANISOTROPY ($R) AND A HIGH NORMAL ANISOTROPY AND HAVING A MANGANESE CONTENT IN THE RANGE OF 0.03 TO 0.025. PERCENT, COMPRISING: COLD REDUCING WITH
 2. A method as claimed in claim 1, wherein after annealing the steel is subjected to temper rolling.
 3. The method of claim 1 wherein said reduction rate is 75 percent.
 4. The mEthod of claim 1 wherein said reduction rate is 80 percent.
 5. The method of claim 1 wherein said reduction rate is 84 percent. 